1. Field of the Invention
The present invention relates to a method of recycling molded plastic parts for photosensitive material and to recycled plastic molded parts for photosensitive material.
2. Description Related to the Prior Art
Conventionally, molded plastic parts are used for example as a camera body of a lens-fitted photo film unit. The camera body contains a film cartridge as a photosensitive material, and simple photo mechanisms therein.
Almost all of the molded plastic parts of the camera body are used in order to pack the film cartridge in a light-tight fashion. Accordingly, they have ever been formed of a composition of thermoplastic resins. Hereafter, the molded plastic parts used for photosensitive material are simply called molded plastic parts.
The molded plastic parts must have following qualities and properties:
1. perfect light-shielding properties,
2. high temperature resistance and high weather resistance, that is, a shape and a size of the molded plastic part is not changed under temperature variations in use for a long time,
3. sufficient physical strength,
4. inflammability,
5. ease of formation and rareness of molding defect,
6. decreasing the cost,
7. no bad influence on human body.
When the qualities and effects 1–5 are not enough, there will be a bad influence on photographic characteristics, for example, fogging on a photo film, abnormality of photosensitivity, abnormality of color, and uneven density.
There are some reasons for the bad influence on the photographic characteristics. If a shape of the molded plastic part changes and the molded plastic part is defectively molded, the photo film is excessively pressed, which causes pressure fog and scratch on the photo film. Further, when the molded plastic part takes charge, static electricity on the molded plastic part causes a static mark (a branch-shaped and hazily circular fog). Not only such physical defects also chemical substances cause the bad influence on the photographic characteristics. The molded plastic part is often resolved into gases and the gases react to the photo film.
In order to obtain the properties and effects 1–5, an adequate resin is selected and used as a base of the molded plastic parts, and additives, for example light-shielding materials and the like, are added to the resin. Polystyrene resin is normally used. Polystyrene resin is cheap and therefore produced so much. And a shape and a size of the resin-formed part made of the polystyrene resin hardly changes. Further, carbon black is as the light-shielding material in use.
However, the normal polystyrene resin has defects, such as easiness to be eroded by some of oils or organic solvents, low temperature resistance, low weather resistance, insufficient physical strength and easiness to take charge. In order to increase physical strength, a rubber-like material is added to the polystyrene resin. The polystyrene resin containing the rubber-like material is a graft copolymer of styrene monomer and the synthetic rubber having impact strength, such as butadiene rubber and the like.
In order to compensate for another defects, lubricant, antioxidant, ultraviolet absorber, nucleus making agent, aging resistor, antistatic agent, and a dispersing agent. The lubricant increases the flowability of the thermoplastic resin and makes it easily molded. The antioxidant prevents the thermal deterioration of the thermoplastic resin and other additives, and the ultraviolet absorber prevents the photo deterioration of the thermoplastic resin. The nucleus making agent increases the Izod impact strength and abrasion resistance. The aging resistor has an effect not to decrease the physical strength of the thermoplastic resin in accordance with deformation for a long time. Further, the dispersing agent disperses the light-shielding material in the resin to increase the light-shielding properties.
The molded plastic part is molded by using a new pure plastic pellet. As the new pure plastic pellet is heated and melted, the additives partly disperse and react to other substances. Accordingly, adequate amount of the additives are added into the new pure plastic pellet, considering amount of the lost additives. Thus, the new molded plastic part can keep the predetermined effects and properties.
In order to protect the environment and decrease amount of the industrial wastes, industrial products are recycled. The molded plastic parts are withdrawn, crushed, melted to form it into strand-shape with an extruding machine, and cut into a pellet of a predetermined size. The recycled plastic pellet is mixed in the new pure plastic pellet in a production line, and the resin-formed parts are produced.
However, an examination shows that photographic characteristics of the recycled plastic molded part are less than that of the new molded plastic parts. Especially, the photo film in the recycled plastic molded parts is more fogged. The deterioration of the photographic characteristics becomes more obvious in the photo film of high photosensitivity. Accordingly, it becomes more problematic recently, as the photo film of high photosensitivity is used.
In recycling processes, the composition of resin is heated and melted to generate a molding material. Heating the molding material makes the deterioration of heat and the reactions of antioxidants for inhibiting the deterioration of heat proceed. This would be the reason for lower quality of the recycled plastic molded parts than that of the new ones.